Rocking chair or insertion secondary batteries are a type of energy storage device in which carrier ions, such as lithium, sodium or potassium ions, move between an anode electrode and a cathode electrode through an electrolyte. The secondary battery may comprise a single battery cell, or two more battery cells that have been electrically coupled to form the battery, with each battery cell comprising an anode electrode, a cathode electrode, and an electrolyte.
In rocking chair battery cells, both the anode and cathode comprise materials into which a carrier ion inserts and extracts. As a cell is discharged, carrier ions are extracted from the anode and inserted into the cathode. As a cell is charged, the reverse process occurs: the carrier ion is extracted from the cathode and inserted into the anode.
FIG. 1 shows a cross sectional view of an electrochemical stack of an existing energy storage device, such as a non-aqueous, lithium-ion battery. The electrochemical stack 1 includes a cathode current collector 2, on top of which a cathode layer 3 is assembled. This layer is covered by a microporous separator 4, over which an assembly of an anode current collector 5 and an anode layer 6 are placed. This stack is sometimes covered with another separator layer (not shown) above the anode current collector 5, rolled and stuffed into a can, and filled with a non-aqueous electrolyte to assemble a secondary battery.
The anode and cathode current collectors pool electric current from the respective active electrochemical electrodes and enables transfer of the current to the environment outside the battery. A portion of an anode current collector is in physical contact with the anode active material while a portion of a cathode current collector is in contact with the cathode active material. The current collectors do not participate in the electrochemical reaction and are therefore restricted to materials that are electrochemically stable in the respective electrochemical potential ranges for the anode and cathode.
In order for a current collector to bring current to the environment outside the battery, it is typically connected to a tab, a tag, a package feed-through or a housing feed-through, typically collectively referred to as contacts. One end of a contact is connected to one or more current collectors while the other end passes through the battery packaging for electrical connection to the environment outside the battery. The anode contact is connected to the anode current collectors and the cathode contact is connected to the cathode current collectors by welding, crimping, or ultrasonic bonding or is glued in place with an electrically conductive glue.
During a charging process, lithium leaves the cathode layer 3 and travels through the separator 4 as a lithium ion into the anode layer 6. Depending upon the anode material used, the lithium ion either intercalates (e.g., sits in a matrix of an anode material without forming an alloy) or forms an alloy. During a discharge process, the lithium leaves the anode layer 6, travels through the separator 4 and passes through to the cathode layer 3. The current conductors conduct electrons from the battery contacts (not shown) to the electrodes or vice versa.
Existing energy storage devices, such as batteries, fuel cells, and electrochemical capacitors, typically have two-dimensional laminar architectures (e.g., planar or spiral-wound laminates) as illustrated in FIG. 1 with a surface area of each laminate being roughly equal to its geometrical footprint (ignoring porosity and surface roughness).
Three-dimensional batteries have been proposed in the literature as ways to improve battery capacity and active material utilization. It has been proposed that a three-dimensional architecture may be used to provide higher surface area and higher energy as compared to a two dimensional, laminar battery architecture. There is a benefit to making a three-dimensional energy storage device due to the increased amount of energy that may be obtained out of a small geometric area. See, e.g., Rust et al., WO2008/089110 and Long et. al, “Three-Dimensional Battery Architectures,” Chemical Reviews, (2004), 104, 4463-4492.
New anode and cathode materials have also been proposed as ways to improve the energy density, safety, charge/discharge rate, and cycle life of secondary batteries. Some of these new high capacity materials, such as silicon, aluminum, or tin anodes in lithium batteries have significant volume expansion that causes disintegration and exfoliation from its existing electronic current collector during lithium insertion and extraction. Silicon anodes, for example, have been proposed for use as a replacement for carbonaceous electrodes since silicon anodes have the capacity to provide significantly greater energy per unit volume of host material for lithium in lithium battery applications. See, e.g., Konishiike et al., U.S. Patent Publication No. 2009/0068567; Kasavajjula et al., “Nano- and Bulk-Silicon-Based Insertion Anodes for Lithium-Ion Secondary Cells,” Journal of Power Sources 163 (2007) 1003-1039. The formation of lithium silicides when lithium is inserted into the anode results in a significant volume change which can lead to crack formation and pulverisation of the anode. As a result, capacity of the battery can be decreased as the battery is repeatedly discharged and charged.
Various strategies have been proposed to overcome the challenges presented by the significant volume changes experienced by silicon anodes as a result of repeated charge and discharge cycles. For example, Bourderau et al. discloses amorphous silicon (Bourderau et al., “Amorphous Silicon As A Possible Anode Material For Li-Ion Batteries,” Journal of Power Sources 81-82 (1999) 233-236)). Li et al. discloses silicon nanowires (Li et al., “The Crystal Structural Evolution Of Nano-Si Anode Caused By Lithium Insertion And Extraction At Room Temperature,” Solid State Ionics 135 (2000) 181-191. In NL1015956, Sloe Yao Kan discloses a porous silicon electrode for a battery. Shin et al. also disclose porous silicon electrodes for batteries (Shin et al., “Porous Silicon Negative Electrodes For Rechargeable Lithium Batteries,” Journal of Power Sources 139 (2005) 314-320.
Monolithic electrodes, i.e., electrodes comprising a mass of electrode material that retains its a shape without the use of a binder, have also been proposed as an alternative to improve performance (gravimetric and volumetric energy density, rates, etc) over particulate electrodes that have been molded or otherwise formed into a shape and depend upon a conductive agent or binder to retain the shape of an agglomerate of the particulate material. A monolithic anode, for example, may comprise a unitary mass of silicon (e.g., single crystal silicon, polycrystalline silicon, or amorphous silicon) or it may comprise an agglomerated particulate mass that has been sintered or otherwise treated to fuse the anodic material together and remove any binder. In one such exemplary embodiment, a silicon wafer may be employed as a monolithic anode material for a lithium-ion battery with one side of the wafer coupled to a first cathode element through a separator, while the other side is coupled to a second cathode element opposing it. In such arrangements, one of the significant technical challenges is the ability to collect and carry current from the monolithic electrode to the outside of the battery while efficiently utilizing the space available inside the battery.
The energy density of conventional batteries may also be increased by reducing inactive component weights and volumes to pack the battery more efficiently. Current batteries use relatively thick current collectors since the foils that make up the current collectors are used with a minimum thickness requirement in order to be strong enough to survive the active material application process. Advantages in performance can be anticipated if an invention was made in order to separate the current collection from processing constraints.
Despite the varied approaches, a need remains for improved battery capacity and active material utilization.